1. Field of the Invention
The present invention relates to a composite passivation film deposited over a semiconductor device, which is deposited at a low temperature below 150.degree. C.
2. Description of the Prior Art
Hydrogenated amorphous silicon nitride (a-SiN.sub.x :H) film fabricated by plasma-enhanced chemical vapor deposition (PECVD) at about 250.degree. C.-350.degree. C. is widely used as the final passivation layer for microelectronic devices due to its ability to prevent scratches and protect the devices from outside contaminants such as sodium and potassium ions and moisture. For example, at the final stage for fabricating an amorphous silicon hydrogen thin-film transistor (a-Si:H TFT), a heat- and water-resistant a-SiN.sub.x :H film deposited by PECVD is usually coated on the a-Si:H TFT to prevent the transistor from degradation due to percolation of moisture and of salts in the moisture into the interior a-Si:H active interface, thus maintaining the electric properties of the transistor.
However, in certain circumstances, it s not suitable to grow an a-SiN.sub.x :H cassivation layer at such high temperatures as 250.degree. C.-350.degree. C. For a semiconductor device Having an a-Si:H interface, such as an a-Si:H thin-film transistor and an a-Si:H solar cell, it has been reported that the diffusion between the metal source/drain and a-Si:H interface initiates at temperatures below 180.degree. C. Aluminum and a-Si:H even diffuse at about 150.degree. C. Relative descriptions can be found in Haque, et al. J. Appl. Phys. 75, 3928 (1994; Ishihara, et al., Thin Solid Films, 155, 325 (1987); and Ishihara, et al., J. Appl. Phys. 53, 3909 (1982). Furthermore, for the highest electrically conductive gold (Au) contacts, the interaction of Au/a-Si:H begins at an even lower temperature of 130.degree. C. (Hentzell, et al., Appl. Phys. Lett. 50, 933 (1987)).
Therefore, when aluminum or gold is used as the contact metal, if a-SiN.sub.x :H film is deposited at a high temperature (250.degree. C.-350.degree. C.), then diffusion between silicon and metal will occur, and the electric properties of the semiconductor device will be degraded. For the reasons above, to the semiconductor device having the a-Si:H interface, if aluminum or gold is used as the contact metal, it is not suitable to deposit a-SiN,:H passivation film at such high temperatures as 250.degree. C.-350.degree. C.
Hence, researchers have sought to deposit a-SiN.sub.x :H passivation film at a temperature below 150.degree. C. However, it has been found that the lower-temperature (15.degree. C.) deposited a-SiN.sub.x :H film tends to be oxidized by the moisture (water) in the air, and finally oxidized into a-SiO.sub.x :H (Liao, et al., Appl. Phys. Lett., 65, 2229 (1994); and Liao, et al., J. Appl. Phys. 80 (2), Jul. 15, 1996). That is because the lower-temperature (15.degree. C.-150.degree. C.) deposited PECVD a-SiN.sub.x :H material has a looser porous structure. Therefore, when exposed to the air, moisture from the air will gradually percolate through numerous microvoids and microchannels and eventually oxidize the whole film in a layer-by-layer fashion. Also, the oxidation rate increases when the deposition temperature decreases. For example, the 880 nm thick PECVD a-SiN.sub.x :H film grown at 150.degree. C. tends to be totally oxidized within one month, and the 910 nm thick PECVD a-SiN.sub.x :H film grown at a still lower temperature of 100.degree. C. is oxidized even more quickly (within one day the whole film was oxidized) Liao, et al., Appl. Phys. Lett. 65 (17), Oct. 24, 1994.
When a substrate is deposited with a-SiN.sub.x :H material by PECVD, the reaction radicals directly fall onto and randomly deposited on the substrate. Therefore, if the a-SiN.sub.x :H film is deposited at a higher substrate temperature, higher moving energy is provided to these radicals such as SiH2.multidot. and NH.multidot.); thus, the radicals have higher surface mobility with which to find the most suitable place for binding. However, if a-SiN.sub.x :H is deposited at a lower substrate temperature, the film has a looser and more porous structure. Consequently, the water molecules in the air more easily percolate into the film to oxidize it.
Therefore, if a low-temperature (&lt;150.degree. C.) deposited a-SiN.sub.x :H film is used as the protective layer for the semiconductor device, it will be quickly percolated by the water molecules in the air and be oxidized, thereby adversely affecting the electric properties and stability of the semiconductor devices.